Porous inorganic membranes

ABSTRACT

A composite membrane for microfiltration comprises a woven metal mesh support and films bridging the interstices comprising inorganic particles, e.g. zirconia, bonded together by 1-25% of an inorganic adhesive. Group IV metal e.g. Si or Zr phosphate adhesives are preferred. Low temperature film formation without sintering permits the use of non-refractory metal supports.

This invention concerns porous inorganic membranes. Such membranes arewell known and widely used for filters, catalyst supports and otherpurposes. They have various advantages over organic membranes, includingchemical inertness, thermal stability, high flux and uniform pore size.

EPA 348041 published Dec. 27, 1989 describes and claims a compositemembrane comprising an inorganic support having interstices, and porousinorganic films of sintered non-metallic particles carried by thesupport and bridging the interstices. The support may be a mesh offibres, particularly a woven metal mesh. Interstices are distinguishedfrom pores and are defined as having diameters greater than 5 μm andlengths preferably less than 10 times their diameters. The intersticesare bridged by porous films of inorganic material carried by thesupport, preferably substantially coplanar with the support, the minimumfilm thickness across the interstices being not more than about twicethe thickness of the support. Preferably the films bridging theinterstices have concave menisci.

These composite membranes have interesting properties. Because thesupports are inherently flexible and because the films bridging theinterstices are in longitudinal compression at ambient temperature, thecomposite membranes can be bent, often sharply without serious loss offilter performance. By virtue of their metal or other support, themembranes are also much easier to fix in position than prior membraneswith ceramic supports.

These composite membranes may be prepared by dipping a woven metal meshor other support in a slurry of the inorganic non-metallic particles, soas to create films bridging the interstices, and drying and heating thefilms to a temperature sufficient to partly sinter the particles. Thetemperature required for partial sintering must not be so high as todamage the support. This may place limitations, either on the materialof the support, or on the nature and size of the inorganic particlesused and hence on the pore size of the composite membrane. There is aneed for porous membranes or films of inorganic particles that do notrequire excessively high temperatures in their preparation.

It is an object of this invention to provide porous inorganic membranes,based on particles of inorganic material, that do not require heating tothe temperatures necessary to bond the particles by sintering alone.This object is achieved by the use of inorganic adhesives. A differencebetween the use of inorganic adhesives and sintering is that withinorganic adhesives there is essentially no long range transport ofmaterial of the particles.

The invention provides a composite membrane comprising a support havinginterstices of diameter greater than 5 μm and length less than ten timestheir diameters, and porous inorganic films carried by the support andbridging the interstices thereof, wherein the films comprise more than50% by weight of particles of an inorganic material bonded together bymeans of less than 50% by weight of an inorganic adhesive. Preferablythe films are substantially coplanar with the support. Preferably thefilms bridging the interstices have concave menisci. Preferably thesupport is inorganic, although organic polymers are possible. Preferablythe support is a woven metal mesh, although glass and other refractoryfibres can be used. Preferably the support is as described in theaforesaid EPA 348041.

The particulate inorganic material may be chosen to achieve desiredproperties, e.g. chemical and thermal resistance, in the membrane.Suitable materials include refractory materials such as alumina,magnesia, beryllia, zirconia, thoria, titania, chromium oxide, tinoxide, silica, silicon carbide, silicon nitride, boron nitride, boroncarbide, titanium carbode, titanium nitride, chemical stoneware,chemical porcelain and zeolites; also carbon and silicon and metals suchas aluminium.

The pore size of the porous films is related to the particle size. Largeparticles, above about 1 μm in diameter, tend to result in a pore sizeof approximately 10% of the particle diameter. Sub-micron particles maytend to give a pore size which is about 50% of the particle diameter.Particle size is not critical although the membranes of the inventionare likely to be less useful if particles having average diameters below0.1 μm or above about 50 μm are used. The particles may be of uniformsize or alternatively graded. One preferred combination comprises 60 to95% by weight of relatively larger particles of average size 0.5 to 50μm, the balance being of much smaller particles having an average sizein the range 4 nm up to 1 μm but not more than 0.1 times the size of thelarger particles. This invention is partly concerned with membranes formicrofiltration in which the average pore size is from 0.02-5 μm,particularly 0.05-1.0 μm.

The porosity of the porous films is due to the fact that the packingdensity of the particles is less than 100%. The inorganic adhesive neednot itself be porous (although preferred adhesives are porous afterheating, see below). The proportion of inorganic adhesive in the porousfilm is less than 50%. preferably 1 to 25% together with 75 to 99% ofthe particles, sufficient to bond the particles together withoutblocking the pores between them.

So far as inorganic adhesives are concerned, reference is directed toAdhesive Handbook 2nd Ed. Van Nostrand Reinhold 1977, pages 117-138,Chapter 6 by J. H. Wills entitled "Inorganic Adhesives and Cements".Preferred inorganic adhesives according to this invention set by achemical reaction between two or more components thereof (other thanwater), e.g. by an acid-base reaction.

An acid-base cement develops via reactions between an acid and a base.The acid can be mineral (e.g. phosphoric acid), Lewis (e.g. magnesiumchloride or sulphate) or even just an organic chelating agent (e.g.polyacrylic acid). The bases are generally metal oxides such asmagnesium oxide, gel-forming silicate materials such as wollastonite, oran acid decomposable aluminiosilicate-type glass.

Commercial inorganic adhesives are based on silicates and phosphates,soluble silicates being the primary adhesives. Common ones are mainlythe alkali metal (sodium, lithium, potassium) silicates which after heattreatment show good chemical resistance.

The phosphate materials are described in the following reference:Chapter 9 in "Concrete Science", published by Heydon 1981, ISBN-095501-703-1, Eds. V. S. Ramachandron, R. F. Feldman, J. J. Beaudoin.They are based mainly on the following types:

1. zinc-phosphate

2. silicate-phosphoric acid

3. oxide-phosphoric acid

4. acid phosphate

5. metaphosphate/poly(phosphate)

The bonding action in all cases is due to formation of acid phosphates.The adhesives may contain alumina to impart high temperature resistance.

Some inorganic adhesives are well known. A range based on alumina,zirconia and magnesia are sold by ECS. Co. Inc. and by Aremco ProductsInc., both of New York. The use of phosphate-based inorganic adhesivesis described in the following patent publications: DD 247128; DD 201044;JP 82057558; U.S. Pat. No. 3,999,995; U.S. Pat. No. 4,288,346.

According to the invention, the inorganic adhesive is preferablyphosphate-based, since these show excellent temperature and chemicalresistance. Particularly preferred are refractory adhesives based on agroup IV phosphate. Silicon orthophosphate (Si[HPO₄ ]₂) is readilyprepared by mixing a silica gel with phosphoric acid. The orthophosphateis not stable on heating and readily forms silicon pyro-phosphate (SiP₂O₇) on heating above 200° C. Silicon pyrophosphate is highly resistantto chemical attack. It is unaffected by strong acids including hotconcentrated sulphuric acid and 40% aqueous hydrofluoric and is onlyslowly attacked by hot concentrated alkali (N. H. Ray "InorganicPolymers", Academic Press, Chapter 6 pages 141 to 143). Other group IVmetals which form refractory pyrophosphates with similar properties andalso suitable for use as adhesives in the present invention, are Zr, Sn,Ge, Ti, Hf and Pb.

The membranes and films with which this invention is concerned areprepared from a fluid, generally aqueous, precursor. According to onepreferred method a sol or colloidal dispersion of silica or other groupIV metal oxide is mixed with orthophosphoric acid in equimolarproportions. Depending on concentrations, the resulting sol may set to agel. The sol or gel is diluted and mixed in chosen proportions with thechosen particulate inorganic material. Other conventional additives maybe included in conventional concentrations. In particular, an organicbinder may be needed to provide film-forming properties prior toheating. Most or all volatile and organic components will be removedduring the heating step, with the result that the final film consistsessentially of the particles of inorganic material and the inorganicadhesive.

According to another preferred method, particles of a refractorymaterial are mixed with a solution of a phosphoric acid or an acidsolution of a phosphate salt, under conditions to cause the solution topartly dissolve the particles. The resulting slurry comprises therefractory particles (whose surfaces have been attacked) and a phosphateadhesive (formed in situ). The acid phosphate used preferably has acation such as ammonia or an amine which volatilises off during thesubsequent heating step. In the resulting membrane, the inorganicparticles and the inorganic adhesive have a common cation.

The resulting fluid precursor may be formed into the desired film bystandard techniques. With mesh supports, application is preferablyeffected by dipping, brush coating, roller coating or spraying. It isoften preferable to perform the dipping procedure two or more times, afilm being formed by ambient temperature drying or heat treating betweeneach dip. By this means more reliably complete coverage of allinterstices of the substrate may be achieved.

The resulting films or membranes are then heated to a temperaturerequired to cure the adhesive and perhaps also to drive off any volatileor organic components. Curing temperatures may be as low as 80° C. forsome commercially available inorganic adhesives. It should not generallybe necessary to heat at temperatures above 800° C. Optimum temperaturesdepend on the nature of the support and of the adhesive and are likelyto be in the range 200° to 600° C., particularly when the adhesive isbased on a group IV phosphate. Sintering of the inorganic particles isnot necessary, but may be advantageous to increase strength or adjustpore size.

The lower curing temperatures required as a result of the use ofinorganic adhesives, give rise to several advantages. One is reducedenergy and capital costs. Another is the ability to use supports, e.g.woven mesh supports, that are not themselves refractory. For example, ifcuring temperatures are kept below 500° C. preferably below 450° C., itbecomes possible to use ordinary stainless steel or aluminium wire mesh.Still lower curing temperatures may permit the use of organic supports.

The membranes of this invention are useful substrates for permeable ormicroporous films. Microporous inorganic films may be formed by sol-geltechniques as described in EPA 242208. Permeable or microporous organicfilms may be formed by techniques described in EPA 242209. When suchfilms are formed on one face of membranes supported on a wire or fibremesh, that face may preferably be continuous. In other words, in orderto achieve maximum porosity, it is preferred that the mesh support not"show through" the face of the porous membrane to which the permeable ormicroporous film is applied. In this case, the porous membrane need notbe exactly coplanar with the support. These additional films may makethe composite membranes suitable for ultrafiltration with average poresizes in the range 0.001-0.5 μm.

The following examples illustrate the invention.

EXAMPLE 1

Colloidal silica sols were obtained commercially. Syton X30 sol wasobtained from Morrisons Chemicals, Liverpool, U.K.

To 40 mls of a Syton X30 sol of concentration 342.6 gl⁻¹ were added 30.2mls of concentrated (85%) orthophosphoric acid, until the mixturereached a composition equivalent to SiP₂ O₇ (SiO₂.P₂ O₅). Mixingresulted in an exothermic reaction and the mixture turned turbid. Withinseveral minutes this cleared to result in a clear transparent sol, whichset to a silicon phosphate gel in 0.5 to 2 hours.

14 ml of the mixture was added to 60 mls of deionised water. This wascombined with 90 g of a zirconia powder of mean particle size less thantwo microns, and mixed to form a slurry. The slurry was ball milled forone hour in a polymeric container to break down agglomerates, and brushcoated onto a sheet of acetone degreased woven 100 mesh Inconel 600(nickel-chromium-iron alloy mesh supplied by G. Bopp and Co. Ltd.). Heattreatment was used to chemically bond the coating suspension, accordingto the following schedule:

a. heating at 60° C. per hour to 500° C.

b. holding at 500° C. for 1 hour

c. cooling at 60° C. per hour to room temperature.

Heat treatment resulted in the formation of microporous compositeceramic/metal structure comprising a non-powdery ceramic coatingsuspended within the metal mesh framework. The ceramic coating wasporous to water and exhibited negligible weight loss upon immersion indeionised water for 10 days.

The pure water flux through the membrane was 1 ml min⁻¹ cm⁻² at 70 kPa.It is envisaged that considerable control over the pore size, and henceflux, of the membranes described in this invention, can be effected byvarying the concentration of the silicon phosphate mixture, and theparticle size of the zirconia powder.

Samples of the silicon phosphate gel were calcined at varioustemperatures, 500° C., 600° C. and 700° C., and examined using x-raydiffraction to determine the phases present. In all cases a dry, hard,porous material was formed, in which crystalline silicon pyro-phosphatewas detected.

Similar results were obtained when 100 mesh stainless steel was used inplace of the Inconel 600.

EXAMPLE 2 Silicon Phosphate Bonded Zirconia Composite Membrane

To a commercially available silica sol, Syton X30, (obtained fromMorrisons Chemicals, Liverpool, U.K.) having an equivalent silicaconcentration equal to 342.6 gl⁻¹, was added concentrated (85%)orthophosphoric acid. The amount added was sufficient to give a SiO₂ :P₂O₅ ratio of 1:1. The exact quantities mixed were as follows:

    ______________________________________                                        Syton X30             40     mls                                              Orthophosphoric Acid  30.2   mls                                              ______________________________________                                    

The resulting mixture underwent a mild exothermic reaction, to produce aturbid liquid. Within several minutes this cleared to give a transparentsol. This then set to a clear gel in 0.5 to 2 hours.

While the mixture was a liquid sol, 10 mls was transferred to 120 mls ofdemineralised water. After stirring, 90 g of Zirconia powder, meanparticle size less than 5 μm, was added to form a slurry. The producedslurry had a composition equivalent to:

    ______________________________________                                        ZrO.sub.2            90.0   g                                                 SiO.sub.2.P.sub.2 O.sub.5                                                                          6.4    g                                                 H.sub.2 O            120    mls                                               ______________________________________                                    

Using values of 5.89 gcm⁻³ and 3.32 gcm⁻³ for the densities ofmonoclinic ZrO₂ and silicon pyrophosphate SiO₂, P₂ O₅, (SiP₂ O₇)respectively, the slurry composition expressed in volume percentcorresponds to:

    ______________________________________                                                  Vol % Vol % (excluding water)                                       ______________________________________                                        ZrO.sub.2   11.0    88.4                                                      SiO.sub.2, P.sub.2 O.sub.5                                                                 1.5    11.6                                                      H.sub.2 O   87.5                                                              ______________________________________                                    

The slurry was ball milled for 1 hour in a polymeric container to breakdown agglomerates, and brush coated onto a sheet of acetone degreasedwoven 100 mesh Inconel 600 (supplied by G. Bopp and Co. Ltd). Afterdrying in air, a heat treatment was applied to chemically bond thecoating suspension according to the following schedule:

heating at 60° C. per hour to 500° C.

holding at 500° C. for 1 hour

cooling at 60° C. per hour to room temperature.

Heat-treatment resulted in the formation of a microporous compositeceramic/metal structure, comprising a non-powdery ceramic coatingsuspended within the metal framework. The suspended ceramic menisciconsisted of angular zirconia particles bonded together with a siliconpyrophosphate Phase.

X-ray diffraction studies of the silicon phosphate gel calcined attemperature has shown silicon pyrophosphate (SiP₂ O₇) is readily formedat or above 500° C.

The pure water flux through the prepared membrane was measured at 35, 70and 140 kPa. Measured permeate fluxes were as follows:

    ______________________________________                                        35 kPa            0.48 ml min.sup.-1 cm.sup.-2                                70 kPa            0.97 ml min.sup.-1 cm.sup.-2                                140 kPa           1.88 ml min.sup.-1 cm.sup.-2                                ______________________________________                                    

A gas burst for this membrane was found to be in excess of 500 kPa. Poresize distribution, determined using mercury porosimetry, gave a meanpore size of 0.352 μm.

The resistance to corrosion of the membrane was examined in 4 solutions;demineralised water, 0.1M HCl, concentrated HNO₃ (S.G 1.42), and NH₄ OHsolution pH 10.00. Percentage weight loss of the membrane immersed ineach solution for 7 days (168 hours) are given below. Figures are alsoincluded from a similar experiment in which the curing temperature wasonly 450° C.

    ______________________________________                                                  % Weight Loss of Membrane                                                     Silicon Phosphate                                                                            Sintered                                                       bonded Zirconia                                                                              Zirconia                                             Solution    (500° C.)                                                                          (450° C.)                                                                       (950° C.)                             ______________________________________                                        Demineralised                                                                             0.11        0.28     0                                            Water                                                                         0.1M HCl    0.30        2.90     2.32                                         HNO.sub.3   0.54        0.43     0.09                                         NH.sub.4 OH 0.03        0.64     0.16                                         ______________________________________                                    

Curing at 500° C. gave excellent resistance to corrosion in thesesolutions, comparable to a sintered microporous zirconia membrane.Curing at 450° C. also gave acceptable results.

Similar results were obtained when 100 mesh stainless steel was used inplace of the Inconel 600.

EXAMPLE 3 Zirconium Phosphate Bonded Zirconia Composite Membrane

Into 72 mls of demineralised water 12 g of ammonium dihydrogen phosphate(NH₄ H₂ PO₄) was dissolved. While stirring, 90 g of zirconia powder,having a mean particle size under 5 μm (obtained from UniversalAbrasives), was added producing a slurry. The resulting slurry had ameasured pH of 4.6 and a composition equivalent to:

    ______________________________________                                               ZrO.sub.2     90    g                                                         NH.sub.4 H.sub.2 PO.sub.4                                                                   12    g                                                         H.sub.2 O     72    mls.                                               ______________________________________                                    

The slurry was ball milled for 1 hour in a polymeric container and, asin Examples 1 and 2, brush coated onto a 100 mesh size Inconel 600. Dueto the mild acidic nature of this slurry, compared to Examples 1 and 2,a woven aluminium mesh can be used.

After drying in air, the coated mesh was chemically bonded using aheat-treatment consisting of:

heating at 60° C. per hour to 600° C.

holding at 600° C. for 1 hour

cooling at 60° C. per hour to room temperature.

Heat-treatment results in the formation of a microporous compositemembrane comprising a non-powder ceramic coating supported by a metalmesh framework. The slurry was applied in such a way as topreferentially deposit a thicker layer on one side of the meniscus, toallow the coating to extend beyond the mesh wire radius.

Examination of the ceramic membrane surface shows it to consist ofangular particles chemically bonded, to give a highly porous structure.The pure water flux through the prepared membrane was measured at 35, 70and 140 kPa.

    ______________________________________                                        35 kPa            1.4 ml min.sup.-1 cm.sup.-2                                 70 kPa            3.3 ml min.sup.-1 cm.sup.-2                                 140 kPa           5.4 ml min.sup.-1 cm.sup.-2                                 ______________________________________                                    

Gas burst pressure for this membrane was found to be in excess of 500kPa.

Pore size distribution was evaluated using mercury porosimetry, giving amean pore size of 0.376 μm.

As for Example 2, resistance to corrosion of the membrane was examinedin 4 solutions; demineralised water, 0.1M HCl, concentrated HNO₃ (S.G1.42), and a NH₄ OH solution of pH 10. Percentage weight loss observedfor the membrane immersed in the solution for 7 days (168 hours) aregiven below:

    ______________________________________                                                 Zirconium Phosphate                                                                         Sintered                                                        bonded        Zirconia (950° C.)                              ______________________________________                                        Demineralised                                                                            0.10            0                                                  Water                                                                         0.1M HCl   2.11            2.32                                               HNO.sub.3  0.83            0.09                                               NH.sub.4 OH                                                                              0.16            0.16                                               ______________________________________                                    

Excellent corrosion resistance is observed for the zirconium phosphatebonded microporous membrane.

EXAMPLE 4 Ceramic Cement Bonded Zirconia Composite Membrane

A range of low temperature curing ceramic adhesives is available fromAremco Products Inc. Three cement types were used in preparing ceramiccomposite membranes: a phosphate composition with alumina, Aremco 503; asilicate composition with zirconia, Aremco 516; and a silicatecomposition with magnesium oxide, Aremco 571. Other ceramic cement typesmay be suitable.

10 g of adhesive was added to 60 ml of demineralised water. Afterstirring to thoroughly disperse the ceramic adhesive, 90 g of ZrO₂ wasadded to form a slurry. The zirconia powder had a mean particle sizeunder 5 μm. The slurry was ball milled for 24 hours and brush coatedonto a 100 mesh Inconel 600.

After drying in air, the coated mesh was subjected to a heat-treatmentto cure the ceramic adhesive. A suitable heat-treatment is as follows:

93° C. for 2 hours

121° C. for 1 hour

250° C. for 1 hour

with a slow ramp rate between each temperature, typically 60° C. perhour. The composite membrane formed comprised microporous menisci ofzirconia particles adhered together by the ceramic adhesive, suspendedwithin the Inconel mesh interstices. The prepared membranes wereobserved to be porous to pure water.

Corrosion resistance of each membrane was evaluated in four solutions;demineralised water, 0.1M HCl, concentrated HNO₃ (S.G. 1.42), and a NH₄OH solution of pH 10. The percentage weight loss for each membrane,prepared using three of the ceramic adhesives, is given below for animmersion period of 7 days (168 hours).

    ______________________________________                                                  Ceramic Adhesive                                                    Solution    503          516    571                                           ______________________________________                                        Demineralised                                                                             0.14         0.48   0.54                                          Water                                                                         0.1M HCl    2.67         2.28   2.49                                          Conc HNO.sub.3                                                                            1.14         0.29   0.66                                          NH.sub.4 OH 0.67         0.95   0.25                                          ______________________________________                                    

EXAMPLE 5 Aluminium Phosphate Bonded Zirconia Composite Membrane

Commercially available aluminium dihydrogen phosphate Al(H₂ PO₄)₃ fromAlbright and Wilson, having a specific gravity of 1.55 and 48 wt %phosphate was used. To 54.3 g of this phosphate solution was added 90mls of demineralised water and 90 g of zirconia powder having a meanparticle size of sub 2 μm. This amount being sufficient to give an Al:Pratio of 1:3, and an equivalent solid composition of:

    ______________________________________                                               ZrO.sub.2      90    g                                                        Al(PO.sub.3).sub.3                                                                           10    g                                                        Water          90    ml                                                ______________________________________                                    

i.e. 20 wt % of inorganic binder.

The slurry was ball milled for 1 hour in a plastic container to breakdown aggregates and homogenise the mixture, with a viscosity of ≈100 cpat 20° C. Woven 100 mesh Inconel 600 metal sheet was solvent degreasedin a trichloroethane bath and followed by a thermal treatment at 450° C.for 10 minutes, to produce a clean surface onto which the slurry wasbrush coated.

After drying for 4 hours in air, a heat treatment was applied tochemically bond the coating suspension in the menisci of the woven metalmesh using the following firing schedule:

heating at 60° C./hr to 600° C., isothermal hold for 1 hour and coolingat 60° C./hr to room temperature. A second coat was applied and thefiring schedule repeated to obtain a microporous composite ceramic/metalstructure comprising a non-powdery ceramic coating suspended within themetal framework.

The pore size distribution was obtained by mercury porosimetry. Themodal diameter was 0.2483 μm.

The resistance of the membrane under static conditions for 7 days tochemical attack was tested using 4 solutions namely demineralised water,concentrated nitric acid, 0.1M hydrochloric acid, and ammonium hydroxidesolution in terms of percentage weight loss. It was then compared tothat of a supported zirconia membrane sintered at 950° C.

    ______________________________________                                                 Loss Wt %                                                                     Aluminum Phosphate                                                                          Sintered Zirconia                                      ______________________________________                                        Demineralised                                                                            0.11            0                                                  Water                                                                         0.1M HCl   2.98            2.32                                               Conc. HNO.sub.3                                                                          0.87            0.09                                               NH.sub.4 OH                                                                              5.93            0.16                                               ______________________________________                                    

Aluminium phosphate bonded membrane is not as chemically resistant assintered zirconia in extreme pH conditions. This is because themetaphosphate phase formed at the heat treatment of 600° C. is not asresistant as zirconia to alkali and acids. Chemical resistance couldhave been improved by heating to 900° C.

Pure water flowrates through the membrane were measured at 100 kPa and385 kPa. The measured permeate fluxes were as follows:

    ______________________________________                                        100 kPa           1.47 ml cm.sup.-2 min.sup.-1                                385 kPa           6.55 ml cm.sup.-2 min.sup.-1                                ______________________________________                                    

The experiment was repeated with a different woven metal mesh support.There was used 100 mesh Hastealloy (a Ni, Cr, Fe, Si, Mn, Mo alloy meshsupplied by G. Bopp & Co., London N2). Similar results were obtained.

EXAMPLE 6

To 40 mls of a Syton X30 sol (obtained from Morrisons Chemicals,Liverpool) of concentration 342.6 gl⁻¹ were added 30.2 mls ofconcentrated (85%) orthophosphoric acid, this amount being sufficient togive an SiO₂ :P₂ O₅ ratio of 1:1. The resulting mixture underwent anexothermic reaction, to produce a turbid liquid. Within several minutesthis cleared to give a transparent sol. This then set to a clear gel in0.5 to 2 hours.

While the mixture was a liquid sol, 14 mls was transferred to 60 mls ofdemineralised water. After stirring, 90 g of zirconia powder having amean particle size less than 2 μm was added to form a slurry. The slurrywas ball-milled for 1 hour in a plastic container to break downagglomerates, and brush coated onto a sheet of acetone degreased woven100 mesh stainless steel (supplied by Potter and Soar, Banbury). Heattreatment was used to chemically bond the coating on the stainless steelaccording to the following schedule:

a. Heating at 60° C. per hour to 500° C.

b. Isothermal hold at 500° C. for 1 hour.

c. Cooling at 60° C. per hour to room temperature.

Heat treatment resulted in the formation of microporous compositeceramic/metal structure comprising a non-powdery ceramic coatingsuspended within the stainless steel mesh framework. The ceramic coatingwas porous to water and exhibited negligible weight loss upon immersionin deionised water for 4 days.

The mean pore size as determined by Coulter Porometry was 0.14 μm, andthe pure water flux through the membrane was 1 ml min⁻¹ cm⁻² at 70 KPa.

EXAMPLE 7 Cement Membranes supported on Glass Fibre Filters

A silicon phosphate containing slurry of sub two micron zirconiaparticles was prepared as in Examples 1 and 2. An aluminum phosphatecontaining slurry of sub two micron zirconia particles was prepared asin Example 5.

Glass fibre filters were obtained from Whatman Paper Ltd., Kent, England(grade GF/D, nominal pore size range 4 to 12 micron), and from PoreticsCorporation, Livermore, Calif., USA, type GC-50, nominal pore size 50micron.

The silicon phosphate slurry was brush coated onto samples of bothfilters. The slurry was imbibed by capillary action into the filters,forming a fine filtration layer above and within the glass fibresubstrate. After drying for four hours, the samples were heat treatedaccording to the following schedule:

Ramp up 60° C. per hour to 500° C.

Hold one hour 500° C.

Ramp down 60° C. per hour to room temperature.

The resulting composites were rigid, porous filters. That the filterswere porous was proved by observing water wicking through the compositemembranes.

Similar results were obtained with the aluminium phosphate slurry.

We claim:
 1. A composite membrane comprising a support havinginterstices of diameter greater than 5 μm and length less than ten timestheir diameters, and porous inorganic films carried by the support andbridging the interstices thereof, wherein the films comprise more than50% by weight of particles of an inorganic material bonded together bymeans alternative to sintering, comprising less than 50% by weight of aninorganic adhesive.
 2. The composite membrane as defined in claim 1,wherein the support is inorganic.
 3. The composite membrane as definedin claim 2, wherein the films are of substantially coplanar with thesupport.
 4. The composite membrane as defined in claim 2, wherein thefilms comprise from 75 to 99% of the particles bonded together by meansof 1 to 25% of the adhesive.
 5. The composite membrane as defined inclaim 2, wherein the inorganic adhesive is phosphate based.
 6. Thecomposite membrane as defined in claim 2, wherein the inorganic adhesiveis based on a group IV phosphate.
 7. The composite membrane as definedin claim 2, wherein the particles and the adhesive have a common cation.8. The composite membrane as defined in claim 2, wherein the inorganicadhesive is silicate based.
 9. The composite membrane as defined inclaim 1, wherein the films are of substantially coplanar with thesupport.
 10. The composite membrane as defined in claim 9, wherein theinorganic adhesive is phosphate based.
 11. The composite membrane asdefined in claim 9, wherein the inorganic adhesive is based on a groupIV phosphate.
 12. The composite membrane as defined in claim 1, whereinthe support is a woven metal.
 13. The composite membrane as defined inclaim 1, wherein the inorganic adhesive is phosphate based.
 14. Thecomposite membrane as defined in claim 1, wherein the films comprisefrom 75 to 99% of the particles bonded together by means of 1 to 25% ofthe adhesive.
 15. The composite membrane as defined in claim 1, whereinthe particles have an average diameter in the range of 0.1 to 50 μm. 16.The composite membrane as defined in claim 1, wherein the inorganicadhesive is phosphate based.
 17. The composite membrane as defined inclaim 1, wherein the inorganic adhesive is based on a group IVphosphate.
 18. The composite membrane as defined in claim 1, wherein theparticles and the adhesive have a common cation.
 19. The compositemembrane as defined in claim 1, wherein the inorganic adhesive issilicate based.
 20. The composite membrane as defined in claim 1,wherein the inorganic adhesive is silicate based.